Signal transmission quality of a radio communication system depends on a situation of a radio transmission path. Therefore, received power during a normal operation is set higher than the received power of which minimum quality can be secured. As a result, a fluctuation immunity in the radio transmission path is small in a normal state. However, in the normal state, a modulation scheme of more multiple levels can be selected, which can enhance a transmission capacity.
Meanwhile, even in a state in which the quality of the radio transmission path is low, an instantaneous interruption of a radio channel can be avoided by employing a modulation scheme (of a small number of multiple levels) exhibiting a large fluctuation immunity in the radio transmission path. However, if the modulation scheme exhibiting a large fluctuation immunity in the radio transmission path is employed, the transmission capacity is reduced. Such a scheme for changing the modulation scheme depending on the situation of the transmission path is called “adaptive modulation scheme” in this technical field. By employing the adaptive modulation scheme, it is possible to maximize the transmission capacity while securing the minimum transmission capacity.
That is, the adaptive modulation scheme represents a function of maximizing the transmission capacity while securing the minimum transmission capacity depending on the quality of the radio transmission path. To be described in detail, in the adaptive modulation scheme, if the quality of the radio transmission path is degraded, the modulation scheme is automatically switched over from the modulation scheme (of a large number of multiple levels; hereinafter, referred to as “modulation scheme A”) exhibiting a small fluctuation immunity in the radio transmission path but a large transmission capacity thereof to the modulation scheme (of a small number of multiple levels; hereinafter, referred to as “modulation scheme B”) exhibiting a large fluctuation immunity in the radio transmission path but a small transmission capacity thereof. Thereby, the transmission capacity is reduced, but a probability of occurrence of an interruption of the radio channel or a signal error is lowered. In contrast, if the quality of the radio transmission path is improved, the modulation scheme is automatically switched over from the modulation scheme B to the modulation scheme A to thereby enhance the transmission capacity. Examples of a technology related to this adaptive modulation scheme include one described in JP 57-159148 A (hereinafter, referred to as “Patent Document 1”). Patent Document 1 discloses examples of phase shift keying (PSK) modulation schemes of two phases, four phases, eight phases, sixteen phases, and the like as examples of the modulation scheme.
In a radio communication device using the above-mentioned adaptive modulation scheme, a multi-level modulation scheme higher in frequency use efficiency is used in a case where the quality of the transmission path is high as in good weather and received power high enough can be secured. Meanwhile, in a case where the quality of the transmission path is low as in rainy weather and the quality of a transmission signal is degraded in the multi-level modulation scheme, the radio communication device using the above-mentioned adaptive modulation scheme is switched over to the modulation scheme (of a small number of multiple levels) which exhibits a larger difference between transmission power and the minimum received power of which minimum quality can be maintained (and which exhibits a high system gain).
As described above, in the adaptive modulation scheme, irrespective of the quality of the transmission path, it is possible to achieve an increase in the transmission capacity under normal operating conditions while securing signal transmission higher in priority. At present, the increase in the transmission capacity is demanded from the radio transmission device as an infrastructure of a mobile communication system. As an implementation measure, the adaptive modulation scheme has been becoming important.
The adaptive modulation scheme as described in Patent Document 1 is applied to a burst transmission scheme representing transmission of discontinuous signals. The transmission power in the burst transmission scheme is determined for each modulation scheme corresponding to each burst signal, and hence the quality of the radio transmission path can be judged only based on the received power.
However, in a case where the adaptive modulation scheme for performing switching judgment only based on the received power is applied to the radio transmission device for transmitting/receiving continuous signals, if an instantaneous interruption in the received power occurs due to fading or the like, the modulation scheme is erroneously switched over, leading to a problem that a loss of signal occurs due to the switching.
In addition, in a case where the received power constantly fluctuates with the unstable quality of the radio transmission path, the modulation schemes are continuously switched over to each other, which leads to a problem that continuous losses of signal occur accordingly.
Meanwhile, in the radio communication system, an automatic transmission power control function (automatic transmitter power control (ATPC)) is used as a technology for reducing an interference amount with respect to another radio circuit by dropping the transmission power under normal operating conditions to as low a level as possible while maintaining the quality of the radio transmission path. In the case of using the ATPC, the transmission power is raised only when the received power becomes low due to the rainy weather or the like. Examples of a technology related to the ATPC include one described in JP 2005-236709 A (hereinafter, referred to as “Patent Document 2”). Patent Document 2 discloses examples of a super multi-level modulation schemes such as binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 64-QAM, 256-QAM, or 1024-QAM as the modulation scheme.
In the radio communication system described in Patent Document 2 described above to which the ATPC is applied, optimization of the interference amount can be achieved, but the transmission capacity is constant. Therefore, the transmission capacity is determined by the modulation scheme assumed to be used when the situation of the transmission path is deteriorated.
As a measure of solving the problems as described above, a technology that combines the ATPC and the adaptive modulation scheme is described in JP 2007-214819 A (hereinafter, referred to as “Patent Document 3”). In Patent Document 3, the transmission scheme including the modulation scheme is determined. Patent Document 3 discloses that any one of: a modulation scheme such as BPSK, QPSK, 16-QAM, or 64-QAM; a coding scheme such as convolutional coding, turbo coding, or low density parity check code (LDPC) coding; a transmission scheme using a plurality of antennae such as space time coding (STC) or space division multiplexing (SDM); and the like is used as the transmission scheme.
Further, as described in Patent Document 3, the transmission power is constantly adjusted during its transmission in a case where the ATPC and the adaptive modulation scheme are combined to apply the adaptive modulation scheme to a radio transmission device provided with the ATPC for transmitting/receiving continuous signals. This leads to a problem that the following malfunction occurs when the modulation scheme is switched over only based on the received power.
That is, in the case of performing the switching judgment only based on the received power, when the quality of the radio transmission path is degraded to lower the received power, the modulation scheme is switched over even if the quality of the radio transmission path can be improved by transmission power adjustment using the ATPC, after which the original modulation scheme is regained when the received power is raised by the transmission power adjustment using the ATPC. Accordingly, even in a case of originally taking nothing more than a signal error due to the reduction in the received power, a loss of signal occurs due to the unnecessary switching of the modulation scheme.